1. Field of the Invention
This invention relates to an improvement in the process for removing vinyl chloride monomer from vinyl chloride resin slurry containing the same by blowing steam into the slurry flowing down through the inside of a column provided with perforated plates and downcomers therein. More particularly it relates to a process for preventing the obstacles of the perforated plates such as adhesion of vinyl chloride resin powder onto the lower surface of the plates or clogging of the holes thereof, by washing with hot water, and a hot water washing apparatus therefor.
2. Description of the Prior Art
Vinyl chloride resin slurry is usually produced by polymerizing vinyl chloride monomer in the state of suspension or emulsion in an aqueous medium and after completion of the polymerization, separating and recovering unreacted monomer. The resulting vinyl chloride resin slurry is then subjected to mechanical separation of the aqueous medium, followed by drying by hot air drying or other means to give vinyl chloride resin powder.
However, the above-mentioned separated aqueous medium, exhaust gas during the hot air drying and further the dried vinyl chloride resin powder, each contain vinyl chloride monomer in an extent of amount which is liable to raise a problem by reason of environmental health or is apparently regarded as harmful.
Various processes have been proposed for completely removing vinyl chloride monomer contained in such exhausts or vinyl chloride resin powder or reducing its content down to a harmless extent with respect of environmental health.
Among the processes, a process is known wherein vinyl chloride resin slurry is flowed down from the upper part of a column provided with perforated plates and downcomers; and the slurry is treated with steam blown through small holes in the perforated plates to vaporize vinyl chloride monomer contained in the resin powder or resin slurry along with steam (West German patent applications laid-open Nos. P 2640546 and P 2806694).
Such a process employing perforated plates equipped with downcomers has a merit that the diameter of the small holes is smaller and hence the amount of steam consumed is smaller, over processes employing perforated plates without any downcomer. Further, in the above process of West German patent application laid-open No. P 2806694, side plates are stood on the perforated plates for form a groove-shaped flow path, and the vinyl chloride resin slurry is flowed through the groove-shaped flow path practically in a state of piston-flow, and the process has a merit that the width of retention time distribution can be narrowed.
However, in these steam-treating processes employing a column provided with perforated plates having downcomers, particularly in the process of the above West German patent application laid-open No. P 2806694 (invented also by the present inventors), the following various problems to be solved have been raised;
The small holes of the perforated plates are gradually clogged during the above treatment, and before long, a back flow of vinyl chloride resin slurry through the downcomers occurs, resulting in infeasibility of the treatment.
The present inventors have made strenuous studies for overcoming these problems, and observed that in the case of clogging of the holes of perforated plates, a vermicelli-like mass of vinyl chloride resin powder, protruded upwards from the inside of the holes, is often formed. In view of this fact, the cause of clogging of the holes of perforated plates has been presumed to consist in that vinyl chloride resin particles adhered onto the sidewall constituting the holes or onto the lower surface of perforated plates in the vicinity of the holes, soften, and are gradually sucked toward the holes by steam passing through the holes and further pushed upwards inside the holes, while softening of the particles advances to cause adhesion to each other, resulting in the above-mentioned vermicelli-like mass which clogs the holes.
The present inventors have found that if the lower surface of the perforated plates provided in the above-mentioned column is sufficiently washed with hot water during the steam treatment of vinyl chloride resin slurry in the column, remarkable effectiveness is attained in the solution of the above-mentioned problems.
In addition, the above-mentioned West German patent application laid-open No. P 2806694 discloses a piping for washing the inside of the column, but this shows nothing but a general concept of washing the inside of the column. On the other hand, the present invention discloses that washing of the lower surface of the perforated plates with hot water under specified conditions is notably effective for preventing the holes of the perforated plates from clogging.